linux-kernel - Re: [PATCH v4] drm/i915: Synchronize active and retire callbacks

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Message-ID: <20200420161514.GB1963@sultan-box.localdomain>
Date:   Mon, 20 Apr 2020 09:15:14 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>, stable@...r.kernel.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Matthew Auld <matthew.auld@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] drm/i915: Synchronize active and retire callbacks

On Mon, Apr 20, 2020 at 11:21:42AM +0300, Joonas Lahtinen wrote:
> So it seems that the patch got pulled into v5.6 and has been backported
> to v5.5 but not v5.4.

You're right, that's my mistake.

> In doing that zeroing of ring->vaddr is removed so the test to do mdelay(1)
> and "ring->vaddr = NULL;" is not correct.

I'm not so sure about this. Look at where `ring->vaddr` is assigned:
-------------------------------------8<-----------------------------------------
	ret = i915_vma_pin(vma, 0, 0, flags);
	if (unlikely(ret))
		goto err_unpin;

	if (i915_vma_is_map_and_fenceable(vma))
		addr = (void __force *)i915_vma_pin_iomap(vma);
	else
		addr = i915_gem_object_pin_map(vma->obj,
					       i915_coherent_map_type(vma->vm->i915));
	if (IS_ERR(addr)) {
		ret = PTR_ERR(addr);
		goto err_ring;
	}

	i915_vma_make_unshrinkable(vma);

	/* Discard any unused bytes beyond that submitted to hw. */
	intel_ring_reset(ring, ring->emit);

	ring->vaddr = addr;
------------------------------------->8-----------------------------------------

And then the converse of that is done *before* my reproducer patch does
`ring->vaddr = NULL;`:
-------------------------------------8<-----------------------------------------
	i915_vma_unset_ggtt_write(vma);
	if (i915_vma_is_map_and_fenceable(vma))
		i915_vma_unpin_iomap(vma);
	else
		i915_gem_object_unpin_map(vma->obj);

	/* mdelay(1);
	ring->vaddr = NULL; */

	i915_vma_make_purgeable(vma);
	i915_vma_unpin(vma);
------------------------------------->8-----------------------------------------

Isn't the value assigned to `ring->vaddr` trashed by those function calls above
where I've got the mdelay? If so, why would it be correct to let the stale value
sit in `ring->vaddr`?

My interpretation of the zeroing of ring->vaddr being removed by Chris was that
it was an unnecessary step before the ring was getting discarded anyway.

Sultan